METHOD AND DEVICE FOR BRAKING THE MOVEMENT OF A MELT DURING CASTING IN A MOULD

Abstract: A method and a device for braking the movement of a melt in non-solidified portions of a cast strand during casting of metal. Melt flows into a mould which comprises four copper plates (2, 3) which form a downwardly open casting mould with a substantially rectangular cross section. A static or periodic low-frequency magnetic field (5) is generated across the path of the inflowing melt whereby electric currents are induced in the melt. According to the invention, two opposite copper plates (2) are electrically insulated from each other, and the induced currents are returned via an external return conductor (13) which is connected between the two insulated copper plates (2). The induced currents together with the magnetic field give rise to a force which acts in such a direction that the movement of the melt is braked along the whole width of the mould.

Description: Method and device for braking the movement of a melt during casting in a mould TECHNICAL FIELD The present invention relates to a method and a device for braking the flow of liquid metal, melt, in the non-solidified portions of a cast strand during continuous casting of metals. The melt is braked by generating a static magnetic field adjacent to a mould used for forming the metal. The liquid metal which moves in the magnetic field gives rise to induced currents in the melt, which together with the magnetic field give rise to a braking force. BACKGROUND ART During continuous casting, hot melt flows, directly or through a casting pipe, into a mould. In the mould the melt is cooled and a solidified, self-supporting surface layer is formed before the strand leaves the mould. If incoming melt is allowed to flow into the mould in an uncontrolled manner, it will penetrate deep down into the non-solidified portions of the strand. This renders difficult the separation of particles contained in the melt, which will adhere to the solidification front instead of being separated to the top surface. In addition, the self-supporting surface layer is weakened, which increases the risk of the melt breaking through the surface layer formed in the mould. It is well-known to arrange one or more static or periodic low-frequency magnetic fields in the path of the melt to brake and distribute the inflowing melt. The cast strand is formed by the melt running down into the mould which is open downwards. The cast strand, which after the mould shall have a substantially rectangular cross section, is formed by allowing the melt to flow into a pipe¬ like casting mould, located in the mould, with a correspon¬ ding rectangular cross section. The walls of the casting mould consist of four separate copper plates. Two of the copper plates constitute the long sides of the mould and the other two copper plates constitute the short sides of the mould. By changing the position of the copper plates which constitute the short sides of the mould, the width of the cast strand may be changed. hen the mould is closed, the copper plates are in contact with each other, which means that all potential differences, if any, between the copper plates are equalized. A static or periodic low-frequency magnetic field is generated with permanent magnets or d.c.-fed induction coils, and are adapted to act across the inflowing melt, in parallel with the short sides of the mould. The melt which moves at a velocity (v) in the magnetic field (B) gives rise to induced currents (i) in the melt. The current density and direction of the induced currents in the melt are calculated by means of the following equation: i = v x B (A/mm^) From this follows that the induced current is given a direction which is perpendicular to the plane which is formed by the vectors v and B, that is, substantially in parallel with the long sides of the copper plates. When the induced currents reach the short sides of the mould, these currents are forced to become deflected downwards parallel to the short sides, that is, in the same direction as that in which the melt is moving. The induced currents in the melt together with the magnetic field give rise to a force. The magnitude and direction (F) of the force can be calculated from the following equation: F * = T x B (N/m 2 ) The induced currents which propagate parallel to the long sides of the copper plates give rise to forces which are directed opposite to the direction of the flow of the melt and therefore brake the melt, but at the short sides of the mould, where the induced currents deflect downwards, the force is instead directed outwards towards the short side and hence has no braking effect on the melt. This means that a downwardly directed flow channel is formed along the short sides of the mould, which brings with it slag particles and other unwanted particles down into the cast strand, thus deteriorating the quality of the strand. One object of the invention is to suggest a method and a device for obtaining an even braking of the melt along the whole width of the mould, and to prevent the occurrence of unbraked flow channels along the short sides of the mould. SUMMARY OF THE INVENTION The invention relates to a method and a device for braking the movement of a melt in non-solidified portions of a cast strand during casting of metal. Melt flows into a mould which comprises four copper plates which form a casting mould, open in the direction of casting, with a substantially rectangular cross section. A static or periodic low-frequency magnetic field is generated across the path of the inflowing melt, whereby electric currents are induced in the melt. According to the invention, two opposite copper plates are electrically insulated from each other, and the induced currents are returned via an external return conductor which is connected between the insulated copper plates. The induced currents, together with the magnetic field, give rise to a force which acts in such a direction that the movement of the melt is braked. By using an external return conductor, the induced currents are prevented from being deflected downwards along the short sides of the mould, but instead the induced currents continue straight out through the copper plates. The force which the induced currents produce then becomes directed upwards, against the direction of flow of the melt, along the whole width of the mould. The unbraked flow channels along the short sides of the mould can no longer arise. BRIEF DESCRIPTION OF THE DRAWINGS Figures la-Id schematically show a device for casting of molten metal according to the prior art. Figures 2a-2d schematically show a device for casting of molten metal according to the invention. Figure 3 schematically shows an embodiment of the invention for casting in a plurality of moulds with a substantially square cross section. DESCRIPTION OF THE PREFERRED EMBODIMENTS Figure la shows a device for casting of metal according to the prior art in a horizontal section. A mould 1 comprises four copper plates, two of the plates 2 constituting the short sides of the mould and the other two plates 3 consti¬ tuting the long sides of the mould. When the mould is closed, the copper plates are in contact with each other. Liquid metal, melt, is supplied to the mould through a casting pipe 4. Magnets placed on either side of the melt generate a magnetic field, the flux lines 5 of which are parallel to the short sides of the mould. The magnets each comprise a core 6 and a coil 7 wound around the core. The magnetic field acts around the whole width of the mould. Figure lb shows the device in Figure la in a section A-A. The magnetic field generated by the magnets acts in region 8. The figure shows how an incoming primary flow of melt has been divided up into several flow channels. The flow in the down- wardly directed channels 9 which are arranged adjacent the short sides of the mould is not influenced by the magnetic field and is therefore allowed to continue unbraked down into the melt. The other flows 10 are braked by the magnetic field. Figures lc and Id show the electric currents which are induced in the melt. The currents are induced in a direction which is perpendicular to both the magnetic field and the inflowing melt, that is, parallel to the long sides of the copper plates in a horizontal direction. When the induced currents reach the short sides of the mould, the currents 11 are forced to become deflected parallel to the short sides. The induced currents in the melt, together with the magnetic field, give rise to a force. At the point in the mould where the current is induced parallel to the long sides of the copper plates, the force is directed opposite to the flow direction of the melt and will therefore brake the melt, but at the short sides of the mould, where the induced currents are deflected, the force is directed outwards towards the short side and therefore has no braking effect on the move¬ ment of the melt. In this way, the downwardly directed flow channels 9 arise along the short sides of the mould. Figure 2a shows a device for casting of metal according to the invention. An electrical insulation 12 is arranged between each of the copper plates. The insulation must be of a material which withstands the wear which arises when the copper plates of the short side change position in connection with the width of the cast strand being changed. The insula- tion may, for example, consist of a plastic layer. The insulation allows the short sides of the mould to have different potential. The currents induced in the melt are returned via an external return conductor 13. The return conductor is connected to two opposite copper plates 2 included in the mould, which are parallel to the magnetic field 5. In this way, the induced currents are conducted out through the copper plates on the short sides of the mould, instead of being deflected downwards along the short sides as before. The return conductor 13 comprises an external current source 15. By supplying current to the melt, the induced currents in the melt increase, which increases the braking force. Figures 2c and 2d show the induced currents in the melt. The force which the induced currents produce is directed upwards, against the flow direction of the melt, along the whole width of the mould (Figure 2d) . The unbraked flow channels along the short sides of the mould disappear because of the upwardly-directed braking forces 14. Figure 2b shows the flow of the melt in a device according to the invention. The advantages of connecting an external current source are, for one thing, that the braking force can be increased with an unchanged magnetic field, and, for another, that the braking force may be maintained unchanged with a reduced magnetic field. If a smaller magnetic field is sufficient, the magnets may be made smaller. Figure 3 shows an embodiment of the invention which is applicable to casting in a plurality of moulds at the same time. Four moulds 20a, 20b, 20c, 20d are arranged adjacent to each other. Each one of the moulds comprises two pairs of opposite copper plates 2, 3. The moulds have a substantially square cross section. Between each one of the copper plates in the mould, an electrical insulation 12 is arranged. On each side of the melt, magnets are arranged for generating a magnetic field in the melt. Each magnet comprises a core 6 and a coil 7 wound around the core. Each one of the four moulds are connected in the closed circuit which is formed by the return conductor 13. Each mould (e.g. 20a) is connected in such a way that the return conductor is connected to two opposite copper plates (2) in the mould. The return conductor 13 is common to all four moulds. An external current source 15 is connected to the return conductor 13.

Claims: CLAIMS 1. A method of braking the movement of a melt in non- solidified portions of a cast strand during casting of metal, wherein a mould, comprising four copper plates (2, 3) which form a mould which is open in the direction of casting and has a substantially rectangular cross section, is supplied with an inflowing melt, and a static or periodic low- frequency magnetic field (5) is generated across the path of the inflowing melt, whereby electric currents are induced in the melt, which induced currents together with the magnetic field give rise to forces which at least partially act in such a direction that the movement of the melt is braked, characterized in that the induced currents are returned via an external return conductor (13) which is connected between two opposite copper plates (2) included in the mould, and that the copper plates included in the mould are insulated electrically from each other. 2. A method according to claim 1, characterized in that the induced currents and hence also the braking force in the melt are increased by supplying electric current to the melt via the external return conductor. 3. A method according to claim 1 or 2 during simultaneous casting with a plurality of moulds (20a, 20b, 20c, 20d) , characterized in that the return conductor (13) is common to the moulds. 4. A device for braking the movement of a melt in non- solidified portions of a cast strand during casting of metal, wherein a mould, comprising four copper plates (2, 3) which form a mould which is open in the direction of casting and has a substantially rectangular cross section, is supplied with an inflowing melt, and a static or periodic low- frequency magnetic field (5) is adapted to act across the path of the inflowing melt to induce electric currents in the melt which together with the magnetic field give rise to forces which at least partially act in such a direction that they brake the movement of the melt, characterized in that a return conductor (13) for the induced currents is arranged between two opposite copper plates (2) included in the mould, and that the copper plates included in the mould are arranged electrically insulated from each other. 5. A device according to claim 4, characterized in that the return conductor comprises an external current source (15) . 6. A device according to claim 4 or 5, characterized in that said opposite copper plates (2) are substantially parallel to the magnetic field (5) . 7. A device according to claim 4, 5 or 6 during simultaneous casting with a plurality of moulds (20a, 20b, 20c, 20d) , characterized in that the return conductor (13) is common to the moulds.

Inventors: HALLEFAELT MAGNUS, KOLLBERG STEN, LEHMAN ANDERS, TALLBAECK GOETE, HALLEFAELT, MAGNUS, KOLLBERG, STEN, LEHMAN, ANDERS, TALLBAECK, GOETE
Applicants: ASEA BROWN BOVERI, HALLEFAELT MAGNUS, KOLLBERG STEN, LEHMAN ANDERS, TALLBAECK GOETE, ASEA BROWN BOVERI AB, HALLEFAELT, MAGNUS, KOLLBERG, STEN, LEHMAN, ANDERS, TALLBAECK, GOETE
Application year: 1994
Main classification: B22D11
IPC codes: B22D11, B22D27